The present invention relates to a point-to-multipoint configuration wherein each terminal which is connected to a communication network may differ in the particular attributes it has stored in its memory register, and more particularly to a system for a communication terminal which has an interface which conforms to The ComiteConsultatif International Telegraphiqe et Telephonique (CCITT) Recommendations, I. series, to respond to an incoming call.
The CCITT named a network for offering a digital communication service Integrated Service Digital Network (ISDN) and, in 1984, gave recommendations, I. series (Redbook) on basic ISDN items. These recommendations were published by the International Telecommunication Union in Geneva in 1985 as a 250 page CCITT Redbook, Volume III, Fascicle 111.5, entitled "Integrated Services Digital Network (ISDN)". This publication describes and defines the parameters and service attributes of the ISDN which are all well known and well understood by persons skilled in the telecommunication network field. These service attributes include information transfer attributes, access attributes and general attributes which are listed and defined together with their values in the CCITT recommendations 1.211 and 1.300 of this publication. These service attributes thus pertain to general accessibility of a terminal to a network call and do not include or pertain to specific communication information such as the calling party number of particular user application characteristics. In accordance with the Recommendation I. 451 (CCITT Redbook Vol. III, Fascicle III.5), on receipt of a call accept messages (SETUP) from a network, a terminal compares certain values of its own attributes with those of the service attributes of the call which are contained in the SETUP message and thus makes an attribute compatibility check in order to decide whether or not to respond to the call and, only when it has decided to respond, sends a respond message CONNect (CONN) to the network.
A more specific procedure is as follows.
A message sent from a network to a terminal for connection is called a SETUP message. The SETUP includes various kinds of information elements such as protocol discriminator, call reference, message type, bearer capability, channel identification, progress indicator, terminal capabilities, display, keypad, signal, switchhook, calling party number, calling party subaddress, called party number, called party subaddress, transit network selection, low layer compatibility, high layer compatibility, user-user information (Rec. I.45l, pages 274 and 275). Certain of these information elements, namely bearer capability, and, if necessary, low layer compatibility, high layer compatibility and user-user information are service attributes which may be used for compatibility checking which the terminal performs in order to decide whether it may answer a call arrived (Rec. I.451, pages 288, 307, 308, 314). The terminal has therein a classified table of its own attributes and values, namely the contents of the above-stated bearer capability, low and high layer compatibility and user-user information elements with which it is compatible. The terminal decides whether it may accept an incoming call by comparing the contents of a call message arrived with those stored in this classified table. The remaining information elements listed above are not used or involved in this compatibility checking.
Referring to FIG. 1, assume that a customer premises 20 which is incorporated in an ISDN network 10 includes a point-multipoint bus 22 which is connected to the network 10 via a network termination (NT) 21, that a plurality of terminals (TE) 23, 24, . . . , and 2n are present on the bus 22, and that the terminals 23, 24, . . . , and 2n share a single subscriber number and have no sub-address.
In the case of a prior art response system (FIG. 2) as prescribed by the Recommendation I.451, a SETUP message is broadcast from the network 10 to the terminals 23, 24, . . . , and 2n. As mentioned above, the SETUP includes various kinds of information elements. Among such information elements, the bearer capability, low layer compatibility, high layer compatibility and user-user information are used for compatibility checking which the terminal performs for deciding whether it may answer a call arrived (Recommendation I.451 4.5.5, pages 288-292; 4.5.15 page 307; and 4.5.18 page 308). These information elements other than the bearer capability are omitted, as the case may be. Each of the terminals 23, 24, . . . , and 2n has therein a classified table which is set own attributes and values.
Let us assume that the terminals 23 and 25 are facsimile machines with same attributes and values, and the terminal 25 is added color transfer mode. The code for the color transfer mode is not defined yet. And the internal processing speed of the terminal 25 is slower than that of the terminal 23.
Each of the terminals 23, 24, . . . , and 2n receives the SETUP message and checks the message so as to identify the requested bearer service functionality. If the terminals 23 and 25 find matches between the requested bearer service attributes and their own, each of the terminals 23 and 25 returns a message for wishing to take the call (CONN message) to the network 10. Then the network 10 sends connect acknowledge (CONN ACK) message to that terminal 23 which has returned the CONN message first to allow the terminal 23 and not the terminal 25 to answer the call.
A problem with such prior art response system is that the terminal 23 should always return a CONN message before the terminal 25. Therefore a calling user would be incapable of communicating with a terminal 25 even though it might repeat origination of a call for a number of times.